In-situ phosphorus doping in epitaxial Si and SiGe films or layers using PH3 has been known to demonstrate a very slow incorporation rate of P due to the “poisoning effect” of phosphine on the Si(100) surface. An example of such a doping behavior is shown in FIG. 1 by curve 11. Curve portion 13–14 of curve 11 shows the slow “transient” trailing edge observed in the SIMS profile and corresponds to the slow incorporation rate of P into the silicon film. In FIG. 1 the ordinate represents P concentration in atoms/cc and the abscissa represents depth in angstroms.
The incorporation of P into a Si layer is increased by the addition of a Ge containing gas (7%) along with phosphine in the reaction zone of a UHV-CVD reactor and has been described in U.S. Pat. No. 5,316,958 which issued May 31, 1994 to B. S. Meyerson and assigned to the assignee herein. The phosphorus dopant was incorporated during UHV-CVD in the proper substitutional sites in the silicon lattice as fully electrically active dopants. The amounts of Ge used were small enough that the primary band gap reduction mechanism is the presence of the n-type dopants at relatively high levels instead of the effect of the Ge. In '958, FIG. 2 shows phosphorus being incorporated into a Si layer during UHV-CVD with and without the addition of 7% Ge containing gas. With 7% Ge containing gas, a decade increase in P concentration would be incorporated in 250 to 500 Å into a silicon layer as shown, for example, by the rate of incorporation from 7×1018 atoms/cc to 5×1019 atoms/cc in FIG. 2 of '958.
Another well known problem associated with in-situ phosphorus or boron doping in silicon CVD is its “memory effect” as shown by curve portion 15–16 in FIG. 1 for the case of phosphorus herein which tends to create an undesirable high level of dopant in the background due to its “autodoping behavior”. This “memory effect” is also evident in the SIMS analysis shown in FIG. 1. The “memory effect” corresponds to a very slow fall or decrease in the phosphorus concentration which stems from a residual background autodoping effect. Hence, in-situ doping typically generates a very undesirable “smearing out” of the dopant profile in silicon films formed by CVD.
FIG. 2 shows curve 11 which is the same as shown FIG. 1 and which illustrates the doping profile of the prior art using PH3. Curve 20 shows a desired or targeted profile having a width of 100 angstroms. In FIG. 2, the ordinate represents P concentration in atoms/cc and the abscissa represents depth in angstroms. Curve 11 has a dopant profile of at least 5 times wider or thicker than the targeted profile of 100 Angstroms in width or in depth as shown by curve 20.
As device dimensions shrink and especially for future complementary metal oxide semiconductor (CMOS) logic, MODFET's, and HBT's incorporating SiGe layers, very thin layer structures having a width or thickness of 5–20 nm of high doping P concentrations will be needed which are impossible to obtain with present technology at this point using present ultra high vacuum-chemical vapor deposition (UHV-CVD) or standard silicon CVD processing.